Use pytest fixtures

[pjhartzell]

Since pystac is switching to pytest for testing (#939), we should look at using pytest's fixtures.

[jsignell]

Just a note that the fixtures currently defined in conftest.py start with test_ which is have pretty non-standard naming. I think something like default_item or simple_item or even just item would be more typical.

[gadomski]

Another note: per #1001 (comment), we should find meaningful names for each test fixture that is created from the test cases. Right now the test cases are numbered 1-8, but if we dig in to each test case, we should be able to come up with a more descriptive name that makes it clear what scenario(s) test case is trying to exercise.